Hello huangky,
For difficulties with Safari you'll want to take the steps below:
Connect to a different network
Try to load a website, like www.apple.com, using cellular data. If you don't have cellular data, connect to a different Wi-Fi network and then load the website.
If you're using a VPN (Virtual Private Network), check your VPN settings. If you have a VPN turned on, some apps or websites might block content from loading.
Restart your device
Turn off your device and turn it on again.
• Restart your iPhone
• Restart your iPad
• Restart your iPod touch
Clear website data
You can clear website data occasionally to improve Safari performance.
1. Go to Settings > Safari.
2. Tap Clear History and Website Data.
3. Tap Clear History and Data to confirm.
Turn on JavaScript
Turn on JavaScript if it's not already on.
1. Go to Settings > Safari > Advanced.
2. Find the JavaScript feature.
3. Tap the button to turn on JavaScript.
You can also find those steps in this resource: If Safari isn't loading websites or quits on your iPhone, iPad, or iPod touch - Apple Support
Should that still persist, do you experience the same behavior with any other websites?
If this is happening exclusively with YouTube, you'll want to reach out to them: YouTube Help They'd be in the best position to assist with troubleshooting their particular website.
Take care.